Overview
Properties play a crucial role in Kotlin, serving as a fundamental building block for data encapsulation. Defining properties correctly affects class design, readability, and functionality. Kotlin introduces a concise syntax for declaring properties, along with powerful features like custom getters and setters, backing fields, and delegated properties, making property management both efficient and expressive.
Key Concepts
- Declaration and Initialization: Understanding the syntax for property declaration and the difference between mutable (
var) and immutable (val) properties. - Custom Accessors: The use of custom getters and setters to implement logic during property access.
- Delegated Properties: Advanced property management through delegation, allowing behavior to be added to property operations.
Common Interview Questions
Basic Level
- How do you declare a read-only property in Kotlin?
- How can you initialize a property with a custom getter in Kotlin?
Intermediate Level
- Explain the concept of backing fields in Kotlin. When are they used?
Advanced Level
- Discuss delegated properties in Kotlin. Can you provide an example of a lazy property?
Detailed Answers
1. How do you declare a read-only property in Kotlin?
Answer: In Kotlin, a read-only property is declared using the val keyword. This defines a property that can only be assigned a value once, either at the time of declaration or within the constructor if it is a class property.
Key Points:
- val keyword for read-only properties.
- Immutable after initialization.
- Can be used with custom getters.
Example:
class Person(val name: String) {
val greeting: String
get() = "Hello, $name"
}
This example demonstrates a class Person with a read-only property name and another read-only property greeting with a custom getter.
2. How can you initialize a property with a custom getter in Kotlin?
Answer: A property with a custom getter in Kotlin doesn't store a value itself. Instead, the getter is called every time the property is accessed, and it calculates the value dynamically.
Key Points:
- Custom getters are defined using the get() syntax.
- The property's value is computed each time it is accessed.
- No backing field unless explicitly defined.
Example:
class Circle(val radius: Double) {
val circumference: Double
get() = 2 * Math.PI * radius
}
In this Circle class, circumference is a property with a custom getter that calculates the circumference based on the radius.
3. Explain the concept of backing fields in Kotlin. When are they used?
Answer: A backing field in Kotlin is used to store the value of a property when a custom getter or setter is used. It's referenced using the field identifier within the accessors. Backing fields are necessary when you need to add logic to the getter or setter of a property but still need a way to store its value.
Key Points:
- Used with custom getters/setters.
- Referenced using field in the accessor.
- Automatically available when needed.
Example:
class Counter {
var count = 0
set(value) {
if (value >= 0) field = value
}
}
This example shows a Counter class where the count property uses a backing field (field) to store its value, ensuring it's never negative.
4. Discuss delegated properties in Kotlin. Can you provide an example of a lazy property?
Answer: Delegated properties in Kotlin allow certain common behaviors of properties to be handled by another object. One common use case is lazy initialization, where the property value is computed only upon first access.
Key Points:
- Delegates add behavior to property access.
- lazy is a delegate for lazy initialization.
- Thread-safe by default, but other modes are available.
Example:
val lazyValue: String by lazy {
println("Computed!")
"Hello"
}
In this example, lazyValue is only computed the first time it is accessed, and the initialization block (printing "Computed!" and setting the value to "Hello") is only executed once, demonstrating the lazy initialization behavior.
